1. Field of the Invention
The present invention relates to systems and methods for wireless transmission, and in particular to a system and method for synchronizing transmissions from multiple transmitters.
2. Description of the Related Art
Terrestrial transmission systems have been used for many years to transmit television signals to television sets and other home based receivers. It is desirable for the range and scope of such television transmissions to be well controlled to assure that the television signals are receivable by the maximum number of appropriate viewers. At the same time, such transmissions must prevent interference with other television transmissions, and to prevent reception by inappropriate viewers.
One technique for extending the range of television transmissions is to use multiple transmitters, each disposed in a different location. In some cases, such transmitters are used to provide coverage in areas where a primary transmitter cannot reach. For example, in Los Angeles, many transmitters are placed on Mt. Wilson, where they have a line-of-sight to much of the Los Angeles Basin. However, even when optimally placed, such primary transmitters often cannot provide a strong enough signal to areas that are shadowed by natural features such as mountains, or interfered with by man-made features such as buildings and other transmitters. In such cases, service may nonetheless be provided by use of additional transmitters (known as gap fillers or boosters) disposed proximate the areas unserviced by the primary transmitter. Repeaters or range extenders may also be used to extend the reach of the primary transmitter beyond the typical line-of-sight reception range.
In addition to merely covering areas otherwise unserviced by the primary transmitter, an array of transmitters may be used to strengthen the received signal in some areas, while weakening the signal in others, by use of constructive and destructive interference principles.
One difficulty in the use of such auxiliary transmitters is that the signals that they transmit may interfere with the signals broadcast by the primary transmitter or other transmitter. These problems can be minimized by advantageous placement of the transmitters, by orienting the transmitter antennas in the optimal direction, and by choosing antenna patters such that coverage is maximized while interference is minimized. However, even using such techniques, the coverage area of the transmitters may overlap, raising the specter of interference in those areas.
FIG. 1 is a diagram of a prior art transmission system 100. The transmission system 100 includes a master transmitter 102 that is communicatively coupled a plurality of secondary transmitters 104A-104C (hereinafter alternatively referred to as transmitter(s) 104). The master transmitter 102 may be communicatively coupled with transmitters 104 via a wireless link (including radio frequency (RF), intermediate frequency (IF), microwave, optical frequencies) or a wired link. Wireless communications may be via terrestrial transmission, airborne transmission or by satellite. Further, communications with one of the transmitters 104 may be wireless, while communications with another one of the transmitters 104 may be wired.
The transmitters 106 receive a signal comprising information such as a media program, advertisements from the master transmitter 102 and retransmit the received signal to receiver stations 108A-108D (hereby alternatively referred to as receiver station(s) 108). Each of the transmitters 104 transmits provides a transmitted signal 112A-112C (hereinafter alternatively referred to as transmitted signal(s) 112) in coverage areas 106A-106C (hereinafter alternatively referred to as coverage area(s) 106).
The true coverage area of the transmitters 104 is not as simple as depicted. The actual coverage area will depend on time varying characteristics such as weather, the performance characteristics of the receiver stations 108, and external interference factors such as opaque objects 114 preventing signal passage or reflective objects that direct signals where they are not desired. For example, during periods of extreme weather, coverage areas 106 may be substantially reduced in size, and buildings may reflect signal to undesired areas. Further, a receiver station having improved hardware or software may be capable of receiving a signal from a transmitter in situations where a standard receiver station cannot. Coverage areas 106 include overlapping coverage areas 110A-110B (hereinafter alternatively referred to as overlapping coverage area(s) 110. Receiver stations 108 in overlapping areas (for example, receiver station 108D) may have difficulty receiving a signal because signal provided by transmitter 104A is interfered with by the signal from transmitter 104B. This problem is discussed in further detail in U.S. Pat. No. 7,110,048, issued Sep. 9, 2006 to Weiss, which is incorporated by reference herein.
While the '048 patent proposes a solution to the interference problem, it does not disclose a system that ensures that the data received from a first repeater is completely identical to data received from the main transmitter. If the data received from a first repeater is not identical to that received from the main transmitter regardless of the time delay, the non-identical data will cause mutual interference.
Additionally because the described processing adds potentially different time delays to each of the signals from each transmitter, the '048 patent makes no provision for assuring that the data transmitted by each transmitter is received within the ability of a receiver to equalize out the resultant intersymbol interference.
If the received data is not completely identical, it will seem to a receiver that picks-up both the main and the repeated signal simultaneously as both interference and as an echo. While some of the receivers can equalize out the effects of echoes, they cannot correct for interference. Further, at some point, the delays may be large enough to exceed the capacity of the equalizers to detect and cancel the echoes. Accordingly, the system disclosed in the '048 patent will result in transmissions that cannot be received due to interference or excessively long echoes.
Interference can be from many sources, including multipath and other problems that are caused by the signals 112A and 112B arriving at the receiver at different times. One problem is that the carrier of the first signal 112A will not be in phase with the carrier of the second signal 112B, compromising the demodulation of the signal at the receiver station 108. Another problem is that after demodulation, the data itself from the different transmitters 104 do not arrive at the receiver station 108 at the same time, thus causing reception difficulties.
Channel equalizers in the receiver stations 108 can ameliorate the echoed data to a certain extent, but at some point, the channel equalizers become ineffective, and the echoed data begins to compromise reception of the signal to the point where the data arriving later from other transmitters essentially becomes interference, and resulting data is unusable.
What is needed is a method and apparatus which synchronizes not only the carriers used in the transmission and reception of such data, but also, the data itself. The present invention satisfies that need.